BOSTON (AP) – A woman should be allowed to deduct her $25,000 sex-change operation on her federal taxes because her gender identity disorder was a medical condition, her lawyers argued Thursday.

The U.S. tax agency would not let Rhiannon O’Donnabhain take the deduction, claiming that her sex-change operation was cosmetic, and not medically necessary.

O’Donnabhain is suing the Internal Revenue Service to get the deduction accepted in a potentially precedent-setting case that could affect thousands of people a year in the United States who undergo similar operations.

Both sides rested their cases on Thursday before a tax court judge who will decide the case.

O’Donnabhain, 63, said her case is about more than just money.

“I hope this case sends a clear message that transgender people deserve dignity, respect and equal treatment, not just for our medical care, but in all aspects of our lives, just as every human being deserves dignity, respect and equal treatment,” she said outside of court.

Bennett Klein, one of O’Donnabhain’s lawyers, attacked the testimony of a high-profile government witness who testified that gender identity disorder is not a disease.

He pointed out in court that forensic psychiatrist Dr. Park Dietz had submitted what he called “false testimony” in the 2002 trial of Andrea Yates, a Texas woman who drowned her five children.

Dietz told jurors in that trial that an episode of the television series “Law & Order” depicted a woman who was acquitted by reason of insanity after drowning her children. It was later learned that no such episode existed, and Yates’ conviction was overturned.

Dietz acknowledged Thursday that he had made a mistake, but said his testimony was “erroneous, not knowingly false.”

Dietz testified that for gender identity disorder or any other disorder to be classified a disease, it has to have “an underlying pathological process.”

“There are variations in human nature, and these variations are not diseases,” Dietz said on the stand.

Karen Loewy, a lawyer for the Gay and Lesbian Advocates and Defenders representing O’Donnabhain, said there is medical consensus that gender identity disorder is a “legitimate serious medical condition.”

IRS lawyers were unavailable for comment after the court session.

Attorneys were given a Nov. 6 deadline to file briefs to Judge Joseph Gale. There was no word when he will decide the case.